The invention may notably be applied to the continuous reforming field where a catalyst having the shape of pellets of substantially equal diameter runs through a chain of reactors arranged vertically. At the base of each one of them, the reactors enter a transfer element known as a "lift pot", where they are mixed with a gas under pressure generated during the process and blown through a tube towards an upper inlet of the next reactor. At the end of this chain, the catalyst pellets are directed by a gas stream towards a regenerator and, once regenerated, they are moved again along the chain of reactors. Such a continuous reforming process is for example described in U.S. Pat. No. 4,172,027 filed by the applicant.
During these continuous reforming cycles, a degradation of the catalyst generally occurs along the circuit followed by a phenomenon known as an "attrition". The catalyst is translated into dust depositions on the grates inside the reactors and notably a change in the circulation of the catalyst particles occurs in the pipe connecting the base of each reactor to the blow transfer element. Since these elements work properly only within a narrow range, the flow of blow gas has to be carefully controlled to be proportional to the flow of particles to be transferred. It is useful to control the circulation of the pellets by measuring for example the displacement velocity thereof.
The measurement of parameters relative to the circulation of particles in a pipe may be achieved according to several different techniques.
An acoustic type technique for measuring the displacement velocity of solid particles in a fluid current is used in the coal mining industry. It consists of arranging two pairs of ultrasound transceivers in two different places along a pipe and correlating the signals received respectively by the receivers in response to impulses transmitted by the corresponding transmitters. An example of this technique is described for example in U.S. Pat. No. 4,598,593.
A different technique, also used in the coal mining industry is described for example in U.S. Pat. No. 4,506,541, which allows the density of the particles circulating in pipes to be measured. It mainly consists in transmitting through the pipe an electron beam by means of an isotopic source and measuring, by means of a sensor the energy diffracted by the particles.
Another well-known technique for measuring the velocity of circulating particles such as blood cells is for example described in U.S. Pat. No. 4,596,254. It mainly consists in measuring the frequency shift due to the Doppler effect which affects light waves which have run through the flow of particles and determining the velocity thereof by autocorrelation.